1. Field of the Invention
The present invention relates to a microwave device for a low noise amplifier used in a receiver of a satellite communication system or the like.
2. Related Background Art
A microwave device having a frequency converter is primarily classified to a down-converter and an up-converter. The down-converter has a function to mix a local oscillator output to a microwave to take out and detect a signal component of a lower frequency. On the other hand, the up-converter has a function to mix the local oscillator output to a low frequency signal power to take out and detect a microwave signal component.
Those converters are frequently used as circuits to correct a signal waveform at a relay station and reduce noise when information is transmitted by using the microwave as a carrier.
A microstrip line having a metal thin film deposited on a dielectric material is frequently used in the microwave device. A general structure of the microstrip line is shown in FIG. 1 in which a conductive layer 31 is arranged on a rear surface of a dielectric material 32 having a thickness 41, and a strip conductor 33 having a width 42 is arranged on a front surface of the dielectric material 32, to form a microstrip line.
In such a microwave device, there is a demand to reduce the thickness of the dielectric material 32. By doing so, the following advantages are offered.
First, since the width of the strip conductor 33 may be narrower, the size of the chip may be reduced. Since a characteristic impedance of the microstrip line is represented by a ratio of the width 42 of the strip conductor 33 and the thickness 41 of the dielectric material 32, the strip conductor 33 may be narrower without changing the characteristic impedance if the dielectric material 32 is thinned.
In the microwave device, the transmission line on the microwave device and the strip line must be long in view of the requirement for the wavelength to be processed. Accordingly, the reduction of the width of the strip conductor greatly contributes to the reduction of the chip size of the microwave device.
Secondly, as the dielectric material 32 is thinned, a via-hole which connects the conductive layer 31 and the strip line 33 is shallow and a transmission loss therebetween is reduced. Accordingly, a low noise property is improved.
Since an RF amplifier of the down-converter requires the low noise property, the improvement by thinning the dielectric material is remarkable.
The RF amplifier in the microwave device chip has a plurality of stages of transistors and source terminals thereof that are connected with the conductor on the rear side of the microwave device chip through via holes. Accordingly, the low noise property of the RF amplifier is improved by reducing the transmission loss of the via holes for the connection.
Thirdly, the variations of shapes and dimensions of the via-holes are reduced and the variation of the performance of the microwave device is improved.
Thus, the thinning of the dielectric material 32 is important in improving the performance of the microwave device.
On the other hand, the following problems are newly raised when the dielectric material is thinned.
First, a yield in a thinning process of the dielectric material (substrate) 32 is lowered.
Secondly, since the semiconductor which uses the dielectric material as the substrate is difficult to handle, a yield in a process after the dielectric material has been thinned is lowered.
Thirdly, a transmission loss increases.
As described above, the performance of the microwave device may be improved by thinning the dielectric material 32 but the thickness of the dielectric material 32 cannot be substantially reduced because of the problems described above.